Conventionally, parts having a flange portion and a small diameter shaft portion made of austenite stainless steel have been used for machine parts and the like in the field of aircrafts and nuclear power or the like, and there is a case that significantly excellent toughness and strength are required.
When a so-called stepped forging, that is, forging into the shape having a flange portion and a small diameter shaft portion, is performed, what is required to achieve both toughness and strength is optimization of alloy structure. For example, JP-A-4-190941 (Patent Literature 1) points out a problem of coarsening of a structure due to working heat of only one heating, that is, in a case of forging without reheating during the forging, or a problem of occurrence of non-uniform microscopic structure in a case where reheating is performed during the forging. To solve these problems, Patent Literature 1 discloses a method in which a radial forging machine is applied to perform extend forging of a small diameter portion in two stages or more, not at one time, and to perform extend forging only in one direction.
Moreover, according to JP-A-2003-334633 (Patent Literature 2), as a method of forming a flange portion and a shaft portion with sufficient yield in a short time, a two-shot or four-shot forging method is provided.